1. Scalar vs Vector Quantities in Motion
A scalar quantity has magnitude (size) only. Examples include distance, speed, time, mass, and energy.
A vector quantity has both magnitude and a specific direction. Examples include displacement, velocity, acceleration, force, and momentum.
In circular motion at constant speed, velocity is constantly changing because the direction of travel changes at every single point.
- Do not mix up distance and displacement: distance is total path length, displacement is straight-line distance from start to end with direction.
2. Distance-Time & Velocity-Time Graphs
On a Distance-Time (or Displacement-Time) graph, the gradient represents the speed (or velocity). A horizontal line indicates the object is stationary.
On a Velocity-Time graph, the gradient represents the acceleration.
Crucially, the area under a Velocity-Time graph represents the total distance travelled (displacement). For triangles use 1/2 × base × height; for trapezoids use 1/2(a + b) × h.
- Never calculate the area under a distance-time graph—it has no physical meaning!
- If the curve on a v-t graph is non-linear, you must count squares or use the trapezoid rule to approximate the area.
3. Falling Objects & Terminal Velocity
When an object is released from rest in a fluid (such as air), initial downward force is only weight (W = mg), so it accelerates at g (9.8 m/s²).
As speed increases, upward air resistance (drag) increases in proportion to velocity.
The net force downwards (F_net = W - Drag) decreases, causing downward acceleration to decrease.
Eventually, the upward drag force balances the downward weight (Drag = W). Resultant force is zero, so acceleration is zero. The object now moves at constant maximum speed called Terminal Velocity.
- Examiner alert: 'Acceleration decreases' DOES NOT mean the object is slowing down! It is still speeding up, just at a slower rate until terminal velocity is reached.
Test Your Knowledge
Practice official past paper questions on this topic with step-by-step scoring.